Receivers for receiving and processing signals are well known in the art, for example in the form of a GPS (Global Positioning System) receiver of a GPS system.
The performance of a receiver may be degraded during time intervals in which wideband noise in the frequency band used for the signals which are to be received by the receiver is present in the environment of the device, since this wideband noise may reduce the signal-to-noise ratio (SNR) of the received signals significantly.
The wideband noise can be generated in particular by a communication system transmitter integrated in the same device as the receiver, or by a nearby communication system transmitter external to this device. Such an internal or external communication system transmitter can be for instance part of a GSM (Global System for Mobile communications) transceiver, of a CDMA (Code Division Multiple Access) transceiver, of a US-TDMA transceiver or of a WCDMA (Wideband CDMA) transceiver.
In a GPS system, for example, several GPS satellites that orbit the earth transmit signals which are received and evaluated by GPS receivers. All GPS satellites use the same two carrier frequencies L1 and L2 of 1575.42 MHz and 1227.60 MHz, respectively. GPS signaling is based on a CDMA principle, i.e. the satellites and their signals are separated by the codes used to modulate the two carrier signals. The employed carrier modulation is a BPSK (bi-phase shift key) modulation, in which the carrier frequency phase is shifted by 180 degrees every time a chip changes from zero to one or vice versa.
The modulation of the carrier frequencies L1 and L2 is illustrated in FIG. 1. After a phase shift of 90 degrees, the sinusoidal L1 carrier signal is BPSK modulated by each satellite with a different C/A (Coarse Acquisition) code known at the receivers. Thus, different channels are obtained for the transmission by the different satellites. The C/A code, which is spreading the spectrum over a 1.023 MHz bandwidth, is a pseudorandom noise sequence which is repeated every 1023 chips, the epoch of the code being 1 ms. The term chips is used to distinguish the bits of a modulation code from data bits.
In parallel, the L1 carrier signal is BPSK modulated after an attenuation by 3 dB with a P-code (Precision code), and the L2 carrier signal is BPSK modulated with the same P-code before an attenuation by 6 dB. Before transmission, the two differently modulated parts of the L1 carrier signal are summed again. The L2 carrier signal carries currently only the P-code. The P-code is much longer than the C/A code. Its chip rate is 10.23 MHz and it repeats every 7 days. In addition, the P-code is currently encrypted, and for that reason it is often referred to as P(Y)-code. Decrypting keys needed for using the P(Y)-code are classified and civil users cannot access them. Therefore, only the L1 carrier C/A code is usable in civil GPS receivers.
Before the C/A code and P(Y)-code are modulated onto the L1 signal and the L2 signal, navigation data bits are added to the C/A and P(Y)-codes by using a modulo-2 addition with a bit rate of 50 bits/s. The navigation information, which constitutes a data sequence, can be evaluated for example for determining the position of the respective receiver. The navigation information comprises e.g. precise satellite orbital parameters and clock correction parameters. When a receiver is able to despread a received signal based on the correct modulation code, it can extract and evaluate the navigation data. A GPS signal which is received at a GPS receiver is further modulated due to the Doppler effect and possibly due other higher order dynamic stresses.
The reception bandwidth of a GPS receiver receiving the modulated satellite signals is related to the reception code. For example, if GPS is based on the L1 carrier C/A code, then the signal requires a frequency band of 1575.42 MHz +/−5 MHz. If a P-code capable receiver is used, then the reception band of the GPS receiver is much wider, it is likely to be 1575.42 MHz +/−24 MHz. The actual used GPS reception bandwidth is further related to the actual implementation, and thus the previously mentioned bandwidths are presented for demonstration purposes. The mentioned GPS bandwidth will thus be used in the following only by way of example.
The GPS standard is currently under modernization. One of the main components of the modernization consists in two new navigation signals that will be available for civil use in addition to the existing civilian service broadcast of the L1-C/A code at 1575.42 MHz.
The first one of these new signals will be a C/A code located at 1227.60 MHz, i.e. modulated onto the L2 carrier frequency, and will be available for general use in non-safety critical applications. The new civilian signal at L2, referred to as “L2CS”, will generally be characterized by a 1.023 Mcps (mega chips per second) effective ranging code having a Time Division Multiplex of two ½ rate codes. The L2CS signal will be BPSK modulated onto the L2 carrier, along with the P(Y)-code. This C/A code will be available beginning with the initial GPS Block IIF satellite scheduled for launch in 2003.
The second one of the new signals will be using a third carrier frequency L5 located at 1176.45 MHz. The L5 carrier frequency will be modulated with C/A codes, more specifically with a CL code of 767,250 chips and a CM code of 10,230 chips. The L5 signal will provide a 10.23 Mcps ranging code, wherein it is expected that improved cross correlation properties will be realized. The L5 signal will be message based. It will include an I (In-phase) channel carrying 10-symbol Neumann/Hoffman encoding and a Q (Quadrature) channel carrying 20-symbol Neumann/Hoffman encoding. The I and Q channels will be orthogonally modulated onto the L5 carrier. The L5 signal falls into a frequency band which is protected worldwide for aeronautical radionavigation, and therefore it will be protected for safety-of-life applications. Additionally, it will not cause any interference to existing systems. Thus, with no modification of existing systems, the addition of the L5 signal will make GPS a more robust radionavigation service for many aviation applications, as well as for all ground-based users, like maritime, railways, surface, shipping, etc. The L5 signal will provide significant benefits above and beyond the capabilities of the current GPS constellation, even after the planned second activity frequency L2 becomes available. Benefits include precision approach navigation worldwide, increased availability of precision navigation operations in certain areas of the world, and interference mitigation. The new L5 signal will be available on GPS Block IIF satellites scheduled for launch beginning in 2005.
At the current GPS satellite replenishment rate, all three civil signals, i.e. L1-C/A, L2-C/A and L5, will be available for initial operational capability by 2010, and for full operational capability approximately by 2013.
In particular communication systems operating in the 1900 band, like GSM1900, which are widely referred to as PCS (Personal Communication System), and communication systems operating in the 1800 band, like GSM1800, which are widely referred to as DCS (Digital Communication System), will generate wideband noise in this GPS L1 band of 1575.42 MHz +/−5 MHz, when C/A code supported GPS is used. When new L2 and L5 frequency GPS signals are used, then lower frequency GSM signals, i.e. GSM900 and GSM800, will generate the same wide band noise problem as GSM1800 to the L1 GPS signal.
Measurements show that if no measures are taken, the SNR of a GPS signal received by a GPS receiver degrades by about 2 dB in case a GSM transmitter implemented in the same device uses for transmissions a single slot TX (transmission) mode, and by about 3 dB in case the GSM transceiver implemented in the same device uses for transmissions a dual slot TX mode.
The GPS receiver, however, requires a sufficient SNR of received satellite signals for being able to correctly acquire and track the signal based on its C/A-code and thus to make use of its content. It is better for the performance of the GPS receiver to receive signals with a particularly low SNR than not to receive any signal at all during short time intervals.
Typically in spread spectrum systems, the AGC (Automatic Gain Control) tunes the received information signal level for A/D (analog to digital) conversion based on the noise level. In normal operating conditions, the noise is coming from background noise, which has a constant power level. The problem arises when the noise level rises rapidly and the AGC tries to adjust an incoming signal to a certain appropriate level for an A/D conversion. A quickly varying high noise level can cause saturation in the A/D converter and the amplitude of the signal is clipped. Of the signal is clipped in conversion, some information signal is lost and thus the receiver performance is degraded.
Also external interferences can block a GPS receiver operation completely, in case multiple communication system transmitters are transmitting in the same area at the same time.
The same problem may further occur when a Galileo receiver is used instead of a GPS receiver. Galileo is a European satellite positioning system, for which the beginning of commercial operations is scheduled for 2008. Galileo comprises 30 satellites, which are distributed to three circular orbits to cover the entire surface of the Earth. The satellites will further be supported by a worldwide network of ground stations. It is planned that Galileo will provide ten navigation signals in Right Hand Circular Polarization (RHCP) in the frequency ranges 1164-1215 MHz, using carrier signals E5a and E5b, 1215-1300 MHz, using a carrier signal E6, and 1559-1592 MHz, using a carrier signal E2-L1-E1. Similarly as with GPS, the carrier frequencies E5a, E5b, E6 and E2-L1-E1 will be modulated by each satellite with several PRN codes spreading the spectrum and with data. Thus, GSM transmitters may equally generate wideband interferences in frequency bands employed by Galileo.
Obviously, the performance of a receiver due to transmissions by a communication system transmitter may equally be degraded in a similar situation in case of a type of a communication system transmitter other than a GSM transmitter and/or a type of a receiver other than a GPS receiver or a Galileo receiver.
In U.S. Pat. No. 6,107,960, a method is proposed for reducing cross-interferences in a combined satellite positioning system receiver and communication system transceiver device. A control signal is transmitted from the communication system transceiver to the satellite positioning system receiver, when the communication transceiver transmits data at a high power level over a communication link. The control signal causes the satellite positioning system signals from satellites to be blocked from the receiving circuits of the satellite positioning system receiver, or to be disregarded by the processing circuits of the satellite positioning system receiver.
For a communication system transmitter which is combined in a single device with the satellite positioning system receiver, it has further been proposed to improve the SNR of received satellite signals by adding an external notch-filter to the transmission path of the communication system transmitter. The notch filter, which is arranged after a power amplifier in the transmission path, has a passband frequency range for passing on the frequencies required for the communication system, and a stop band frequency range for attenuating the frequencies required for the satellite positioning system.
For PCS and DCS, the passband frequency range of the notch filter has to be 1710 MHz to 1910 MHz, and in case GPS is used as a satellite positioning system, the stop band frequency range has to be 1558.42 MHz to 1580.43 MHz. In order to improve the SNR of received GPS signals to a useful level, a very high attenuation is required for the stop band. Applying a high attenuation, however, increases also the insertion loss of the notch filter at the pass band of the filter. Due to this additional loss after the power amplifier, more output power has to be taken from the power amplifier, which increases the current consumption.
Measurements show that an antenna isolation of about 10 dB is required for single slot GSM, if the GPS SNR is to be improved to a desired level of 0.5 dB degradation. To a GSM1800 transmission path, a 30 dB external GPS band attenuator has to be added for achieving the same desired level of 0.5 dB degradation. For dual slot GSM, the required attenuation is even higher.
The insertion loss of a GPS notch-filter with a 30 dB GPS band attenuation will lie between 0.7 dB and 1.0 dB. An insertion loss between 0.7 dB and 1.0 dB increases the current consumption of the power amplifier by about 20%, compared to a current consumption without insertion loss.
It is thus a disadvantage of the approach using a notch-filter that an extra component is needed in the communication system transmitter and that the power amplifier current consumption increases about 20%, which also results in an increased heating of the device. On the whole, the costs for improving the GPS SNR by only about 1.5 dB are high.